The present invention relates generally to drawn and ironed containers and more particularly to a process and apparatus for manufacturing such containers. While the disclosure is specifically described in connection with making beer and/or beverage containers, it will have applicability in other drawing and ironing processes.
The drawing and ironing process for making thin walled containers that have a cylindrical sidewall and a unitary or integral end wall has been used in the container manufacturing industry for several years. The most common process utilized in manufacturing containers, particularly containers that are utilized for packaging beer and/or carbonated beverages consists of cutting a disc of predetermined diameter and substantially simultaneously converting the disc into a cup. The cup is then fed into a special press which is generally referred to as a drawing and ironing press or body maker, wherein the cup is initially redrawn to a smaller diameter, the final diameter of the inside of the container, and the wall thickness of the redrawn cup is then reduced through ironing dies or rings that cooperate with a punch that moves the cup through the iron rings. This process is described in detail in an article appearing in the November, 1973 Aerosol Aid Magazine entitled the "Drawn and Ironed Can, Understanding the Technology" by Edward G. Maeder.
In another well known process for making thin walled containers, the disc is initially converted into a cup having an internal diameter equal to the finished container and the side wall thickness of the cup is reduced through ironing rings that cooperate with a punch in a manner similar to that discussed above.
For many years most of the drawn and ironed containers were formed from aluminum. Quite recently, tinplate, which consists of black plate that is coated on both surfaces with a thin layer of tin, has become an acceptable alternate for aluminum in making drawn and ironed containers. While tinplate has been found to be an acceptable substitute and competitive alternate for aluminum, the availability of the tin for this material is limited. Furthermore, manufacturers are constantly striving to reduce the costs of making containers. Thus, recent attempts have been made to form drawn and ironed containers from black plate without the use of the tin coatings thereon.
One problem that has been a significant factor in successfully making drawn and ironed containers, particularly from tinplate or black plate, is stripping the finished container off the punch. Stripping problems relate primarily to shrinking of the container on the punch after the last ironing step and before stripping actually takes place which results in large frictional forces between the punch and the container. Stripping problems are most acute where the temperature gradient between the punch and container is high such as when an operation is initiated with ambient temperature tooling. Thus, many containers must be discarded because they are damaged during stripping. In some instances, during start-up, the containers are not acceptable because they are too short, which results from the wall thickness being too thick due to the cold tooling.
Another of the difficulties that have been encountered in making drawn and ironed black plate containers is that the container tends to seize to the ironing dies during the ironing process. Seizing of the container to the die has been determined to be a function of the pressure and temperature created during plastic deformation of the wall as well as the type of tool material and container material. The pressures required to reduce the wall thickness are determined by the material being used to form the container. Thus, the only parameter that can be controlled is temperature, as the plastic deformation occurs.
In order to reduce the temperature of the container and the tooling during the drawing and ironing process, it has been customary to flow a coolant, which also contains a lubricant, to the surface of the container during the ironing process. Attempts have been made to develop an ideal temperature for the coolant-lubricant.
Attempts have also been made to develop lubricants that can withstand the high pressures and temperatures involved in the drawing and ironing process. One of the problems encountered in developing such lubricants is to maintain the lubricants in the interface between the cup and the ironing dies at the entrance to the die. Heretofore, only coatings, such as organic coatings, phosphates and tin, have been found to have enough viscosity to withstand the shear required to enter the interface between the die and the cup and act as an effective lubricant.